Electrical steel sheets have a high degree of permeability and a low degree of core loss, and are thus frequently used as materials for cores, etc. Electrical steel sheets may be broadly categorized as grain-oriented electrical steel sheets and non-oriented electrical steel sheets.
Grain-oriented electrical steel sheets are characterized by {110}<001> grains having a {110} plane parallel to the rolled surface and a <001> axis (magnetic easy axis) parallel to the rolling direction. Grain-oriented electrical steel sheets have superior magnetic characteristics in a particular direction, and are thus widely used as material for cores of devices that are used at a fixed position, such as transformers, electric motors, generators, or other electric devices. The magnetic characteristics of grain-oriented electrical steel sheets may be expressed by magnetic flux density and core loss. A grain-oriented electrical steel sheet having a higher degree of magnetic flux density and a lower degree of core loss is favored. In general, the magnetic flux density of electrical steel sheets is expressed by B8 values measured in a magnetic field of 800 Amp/m, and the core loss of electrical steel sheets is expressed by W17/50 indicating lost watts per kilogram at 50 Hz and 1.7 Tesla.
N.P. Goss developed an early technique for grain-oriented electrical steel sheets. According to the technique, grains of steel are oriented in the {110}<001> orientation (known as Goss orientation) by a cold rolling method. Thereafter, the technology for grain-oriented electrical steel sheets has been developed up to the present level.
That is, it is necessary to increase the proportion of grains having {110}<001> orientation or a similar orientation so as to manufacture a grain-oriented electrical steel sheet. A heating process is necessary to induce recrystallization of grains of steel sheets, and thus to orient the grains of the steel sheets. In an annealing process, however, the growth of crystals generally occurs in random orientations. Therefore, a particular method is necessary to obtain grain-oriented electrical steel sheets having grains grown in a particular direction.
In general, electrical steel sheets are annealed in two steps: primary recrystallization annealing and secondary recrystallization annealing. Primary recrystallization occurs by using energy accumulated during a cold rolling process as a driving force, and secondary recrystallization occurs by using boundary energy of primarily recrystallized grains as a driving force. During the secondary recrystallization which is also called “abnormal grain growth,” grains grow to a size of several millimeters (mm) to several centimeters (cm).
However, secondarily recrystallized grains have different orientations depending on the temperature of recrystallization. If the secondary recrystallization occurs at a certain temperature, the proportion of grains having Goss orientation increases, and thus an electrical steel sheet having a low degree of core loss may be obtained.
Therefore, it is necessary to suppress the secondary recrystallization until the temperature reaches a certain level at which grains having Goss orientation are obtainable and to start the secondary recrystallization at a certain temperature. Generally, inhibitors are used for this purpose. Inhibitors exist in the form of precipitates in steel and suppress the movement of grain boundaries and the formation of new grains. If a proper inhibitor is selected, the inhibitor may not obstruct the growth of grains at a recrystallization temperature at which the grains recrystallize as grains having Goss orientation, for example, because the inhibitor is dissolved or removed at the recrystallization temperature, and thus the recrystallization and growth of grains having Goss orientation may markedly occur at the recrystallization temperature.
Therefore, the selection of a proper inhibitor may be a crucial factor in increasing the proportion of grains having Goss orientation in electrical steel sheets and reducing the core loss of the electrical steel sheets. An MnS-based inhibitor, developed by ARMCO, USA, may be the first inhibitor. However, in techniques in which MnS-based inhibitors are used, since MnS exists as coarse particles in steel slabs and thus does not function as an inhibitor, MnS is first dissolved through a solid solution treatment and is then precipitated as fine particles. To this end, slabs are heated to 1350° C. or higher to sufficiently dissolve MnS. However, the slab heating temperature is much higher than a temperature to which steel slabs are generally heated and thus may decrease the lifespan of a heating furnace, thereby causing problems such as a decrease in the lifespan of a heating furnace or corrosion of a slab due to silicon oxides melting and flowing on the surface of the slab. In addition, a method of manufacturing non-oriented electrical steel sheets through two cold rolling processes and an intermediate annealing process therebetween has been proposed by ARMCO. However, electrical steel sheets manufactured by the method thereof do not have sufficient magnetic characteristics.
In 1968, Nippon Steel Corporation proposed a new conceptual electric steel sheet product named “Hi-B.” The electric steel sheet product Hi-B uses AlN and MnS as inhibitors and is producible through a single cold rolling process. Although the electric steel sheet product Hi-B has a high degree of magnetic flux density and a low degree of core loss, a slab has to be heated to a high temperature during a solid solution treatment process so as to dissolve inhibitors.
JFE has proposed another electrical steel sheet using MnSe and antimony (Sb) as inhibitors. However, the electrical steel sheet is also disadvantageous in that a slab has to be heated to a high temperature.
To address problems of such high-temperature heating methods of the related art, a low-temperature heating method has been developed. According to the low temperature heating method, inhibitors are not formed at the beginning of a manufacturing process but are formed immediately before secondary recrystallization so that the slab heating temperature may be decreased to 1300° C. or lower, or 1280° C. or lower. The core technology of the low-temperature heating method is a nitriding annealing process in which nitrogen (N) necessary for forming AlN functioning as an inhibitor is added to steel by diffusing nitrogen gas at a later stage of manufacturing. Therefore, a high-temperature heating process is not necessary for dissolving aluminum (Al) and nitrogen (N) and forming AlN. Thus, various process problems of high-temperature heating methods could be solved.
A method of increasing the specific resistance of electrical steel sheets may be considered an important method of decreasing the core loss of electrical steel sheets. That is, as shown in Formula 1 below, the core loss of steel sheets is reverse proportional to the specific resistance of the steel sheets. Thus, particular elements may be added to steel sheets to increase the specific resistance of the steel sheets.Wec=(π2·d2·I2·f2)/(ρ·6)  [Formula 1]
where Wec: core loss, d: crystal diameter, I: current, f: frequency, and ρ: specific resistance.
An exemplary element that increases the specific resistance of electrical steel sheets is silicon (Si). That is, the core loss of electrical steel sheets may be reduced by adding as much silicon (Si) as possible to the electrical steel sheets. However, if an excessive amount of silicon (Si) is added to a steel sheet, the brittleness of the steel sheet is increased, and thus cold-rolling characteristics of the steel sheet are deteriorated. Therefore, the method of adding silicon (Si) has practical limitations. Like silicon (Si), phosphorus (P) may increase the specific resistance of steel sheets. However, since even a very small amount of phosphorus (P) increases the brittleness of steel sheets, there is also a limit to adding phosphorus (P).